A purpose of a brush in an electrical device is to pass electrical current from a stationary contact to a moving contact surface, or vice versa. Brushes and brush holders may be used in electrical devices such as electrical generators, electrical motors, and/or slip ring assemblies, or sliding connection applications, for example, slip ring assemblies on a rotating machine such as a rotating crane or a linear sliding connection on a monorail. Brushes in many electrical devices are blocks or other structures made of conductive material, such as graphite, carbon graphite, electrographite, metal graphite, or the like, that are adapted for contact with a conductive surface or surfaces to pass electrical current.
In some designs, a brush box type brush holder, or other type of brush holder, may be used to support the brush during operation. The brush and brush box may be designed such that the brush can slide within the brush box to provide for continuing contact between the brush and the conductive surface contacted by the brush. Over time, the brush will be reduced in size, or get shorter (i.e., diminish in longitudinal length), for example, as the wear surface of the brush in frictional contact with the conductive surface wears down. Once a brush has worn beyond a threshold amount, a brush may need to be replaced and/or maintenance may need to be performed.